The number of Facebook users in the US will increase 13.4% this year, eMarketer estimates, after 38.6% growth in 2010 and a whopping 90.3% rise the year before. The rate of adult Twitter user adoption has similarly begun to plateau, dropping from 293.1% growth in 2009 to 26.3% this year and still slowing. In many developing countries, these and other networks are seeing their audience growth taper off as most new users come from other countries such as the BRIC nations and Indonesia.

via What Changing Social Media Usage Means for Marketers – eMarketer.